Enumerating Enumerable: Enumerable#cycle

by Joey deVilla on July 6, 2008

Welcome to another installment of Enumerating Enumerable, my series of articles in which I attempt to improve upon RubyDoc.org’s documentation for the Enumerable module. So far, I’ve covered the following methods in this series:

  1. all?
  2. any?
  3. collect / map
  4. count

In this installment, I’m going to cover a method added into Enumerable in Ruby 1.9: Enumerable#cycle. It’s particularly poorly-documented in RubyDoc.org, and there isn’t much written about it anywhere else.

Enumerable#cycle Quick Summary

Graphic representation of the \"cycle\" method in Ruby\'s \"Enumerable\" class.

In the simplest possible terms Given a collection, creates an infinitely-repeating ordered source of its items.
Ruby version 1.9 only
Expects An optional block to act on the items in the infinitely repeating until the break statement is encountered.
Returns An Enumerable::Enumerator that acts as the infinite source of the collection’s items.
RubyDoc.org’s entry Enumerable#cycle

Enumerable#cycle and Arrays

When used on an array and a block is provided, cycle passes each item to the block. Once the last array item has been passed to the block, cycle starts over again at the beginning of the array. cycle goes through the array forever unless it encounters a break statement in the block.

The better use for cycle is when you use it to create an object that spits out the next item in a repeating sequence.

How about one more example? We’ll use Enumerable‘s zip method, an array of dinner items and the “days of the week” cycle object to create a meal plan:

Enumerable#cycle and Hashes

When used on a hash and a block is provided, collect and map pass each key/value pair in the hash to the block, which you can “catch” as either:

  1. A two-element array, with the key as element 0 and its corresponding value as element 1, or
  2. Two separate items, with the key as the first item and its corresponding value as the second item.

Each key/value pair is passed to the block, where the operation in the block is performed on the item. Once the last hash item has been passed to the block, cycle starts over again at the beginning of the hash. cycle goes through the hash forever unless it encounters a break statement in the block.

We can use the same technique that we used with arrays and use cycle to create an object that spits out the next item in a cycling hash. Each time we get an item from the object, it comes in the form of a two-element array where the first element is the key and the second element is the corresponding value.

Let’s try one more example! In this one, we’ll use cycle to create three different enumerators — two made from arrays, one from a hash — to assign cooking chores for the Enterprise-D crew for the next ten days.

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